Toriel

When the “feast” first started toriel thought it was just some illness, so she made a foundation to try and help them but mortality rates were high and her subjects lost faith in her, usually calling her “an evil queen” "the undergrounds killer” this gave toriel depression, due to people's negative views on her and the failing cure to fix them.

When the “feast” had its first major outbreak all the uninfected monsters went straight to toriel, demanding her to speed up the cure and help them, but one day they broke into the castle toriel locked herself away, in the throne room she was forced to listen to monsters crying in pain,children running away from there parents, she couldn't take it anymore so she managed to escape new home and reach the ruin’s locking herself away.

While toriel was locked away in her old home, she made stuffed dolls of her friends and kids she fed them,bathed them and even sung to them, while she slowly mutated and her sanity broke.

One day flowey managed to get inside of toriel’s home he saw, her talking to the dolls so he thought it would be a easy target so he slowly crawled over but toriel managed to notice him, before the attack so she quickly turned around and burned half of his face a permanent reminder, of who he should fear.

After that incident toriel broke out of her house abandoning her doll family and going, on a rampage destroying most of the ruins and its inhabitants creating the maze.

When prey got low in the ruins toriel left looking for bigger prey, this fixed the ecosystem in the ruins and monsters started thriving again, when toriel reached snowdin she encountered sans but both alpha predators stared, at each other for a few minutes before going their separate ways.

When toriel reached new home she made her presence known, by taking down the leaders of the alpha packs and making her new kingdom every monster, fears toriel due to her aggressive nature if one of her subjects, doesn't do what she wants they would be burned alive.